In particular, the invention applies to the embodiment of liquid crystals indicators of the multiplexed type or of the non-multiplexed type or even fluorescent micropoint screens (marked FMS in the continuation of the description) allowing for the display of fixed or animated images.
Various known types of methods exist for controlling matrix display screens.
Matrix display screens comprise a display cell provided with line conductors and crosswise column conductors, one pixel of the screen being associated with each crossing of these conductors.
One description of such an FMS appears in the French patent application No. 87 15432 of the 6th Nov. 1987. In one FMS, the lines correspond to the grids and the columns to the cathodes.
As regards liquid crystal screens, the display material is contained in the display cell. Liquid crystal screens may be multipexed or non-multiplexed controlled.
In more detail and relating to a multiplexed display screen, the line conductors and columns are constituted by column and line electrodes respectively disposed on the internal walls of the cell, one pixel being defined by the zone for overlapping one line electrode and one column electrode.
In the case of a non-multiplexed display screen, the line and column conductors are constituted by addressing lines and control columns which, for example, are disposed on one of the walls of the cell and connected by means of transistors to point electrodes, one d.c. electrode being disposed on the other wall of the cell. According to a further example of this type of screen, the addressing lines and the control columns may be respectively disposed on the internal walls of the cell, the lines being connected by means of transistors to point electrodes and the columns being connected to electrode columns. In these last two cases, one pixel is defined by the zone for overlapping one point electrode with the d.c. electrode or with one column electrode.
Addressing signals are sent onto the various line conductors and control signals are sent onto the column conductors. One example, given by way of illustration and being in no way restrictive, is shown on FIG. 1 and describes such signals where a matrix liquid crystals display screen is controlled by the technique known as the direct multiplexing technique.
For reasons of simplicity and in no way altering the above-mentioned description, this technique is limited in this example to one screen having nine pixels, namely three line conductors L1, L2, L3, and three column conductors C1, C2, C3.
The voltages V1 applied to the line conductors are periodical with a period T known as a frame time or scanning time. For each line conductor, the voltage V1 is equal to a voltage Vmax for a time Ts, known as a line selection time, and is nil, for example, outside this time Ts concerning the rest of the time T. Each line is thus brought successively during a time Ts up to the value Vmax. FIG. 1A shows an addressing cycle of the line conductors. FIG. 1B describes a sequence example of the control voltages Vc applied to the column conductors. Depending on the motif to be displayed, the voltages applied to the column conductors shall be positive or negative.
The values of the voltages applied to the line conductors and column conductors depend on the type of display used.
When the voltage applied to a line conductor is in phase with the voltage applied to a column conductor, the pixel corresponding to their crossing is extinguished (black, for example). If the two voltages are in opposition of phase, the pixel in question is lit up (white, for example).
When the line L1 is otherwise selected when it is brought to Vmax during Ts, the voltage on the column C1 is positive in the example in question. The two column and line voltages are in phase and the pixel corresponding to the crossing of the line conductor L1 with the column conductor C1 is black. When the line L2 is selected, the voltage on the column C1 is negative in the example in question. The two line and column voltages are in opposition of phase and the pixel corresponding to the crossing of the line conductor L2 with the column conductor C1 is white. The state of each pixel is deduced identically.
FIG. 1C gives the display of the screen for the proposed line and column voltages on FIGS. 1A and 1B. The pixels marked N are black and those marked B are white.
For the display of given information and to each corresponding period T, the line and column voltages have their polarity inverted so as to only apply to the display material signals of nil average values.
In the case of a non-multiplexed liquid crystals type screen or an FMS, the selection signals of the line conductors are the same as those shown on FIG. 1A, but they do not undergo any polarity inversion. On the other hand, the signals applied to the column conductors may be either of negative or positive polarity, their amplitude solely depending on the voltage required with the electro-optical effect used.
In all cases, the line selection time Ts depends on the number of line conductors to be selected by the formula Ts=T/M where M is the total number of line conductors and T is the frame time. It is understood that M increases more when the selection time Ts is shorter.
The multipexing rate TM is defined as being the ratio between the frame time T and the selection time Ts of one line conductor. EQU TM=T/Ts
For the known screens, TM=M is established.
When the number of line conductors increases, the multiplexing rate follows this growth and the time Ts diminishes resulting in a reduction of the contrast of a liquid crystals screen and the luminosity of an FMS.
The number of lines currently used in matrix display screens with liquid crystals is about one hundred. Thus, this number is considerably lower than the number of available video line signals which is equal, for example, to about two hundred and eighty at the output of a video recorder.